OBJECTIVES: The secondary structure of nucleic acids is probed by the psoralen photoreaction. Techniques have recently been developed in this laboratory for mapping such secondary structure. The objective of this proposal is to improve these techniques so that they can be used as a high resolution probe for secondary structure. When this objective is realized, the nucleotides in a defined sequence which are base paired in a helical structure will be identifiable. B. The goals for the current year were: 1) the identification of the structural hairpin at the end of the filamentous phage fd; 2) the establishment of a relationship between DNA secondary structure and RNA processing; 3) the mapping of structural loops in 16S E. coli ribosomal RNA in vitro and in the 30S ribosomal subunit.